Field of the Invention
The invention relates to novel polybenzoxazole and polybenzothiazole precursors and to resist solutions containing these precursors.
Throughout the specification and claims, the parenthetical expression (thio) is used as a convenient abbreviation to denote independently at each occurrence the alternative presence of oxygen or sulfur in hydroxyl, mercaptan, ether and thioether groups, as in poly[phenolcarboxamide phenyl ether] and poly[thiophenolcarboxamide phenyl thioether] precursors for polybenzoxazoles and polybenzothiazoles respectively.
In microelectronics, highly heat-resistant polymers are needed as protection and insulation layers. Such polymers can be employed as dielectric between two metal planes, for example in multi chip modules and memory and logic chips, or as buffer coat between the chip and its housing. Some of these polymers, for example precursors of aromatic polyimides (PIs) and polybenzoxazoles (PBOs), have good solubility in organic solvents and good film-forming properties and can be applied to the electronic components by inexpensive spin-coating methods.
The precursors are then cyclized, i.e. converted into the corresponding polymer, by heat treatment, and thus obtain their final properties. ##STR2##
The cyclization is accompanied by disappearance of the polar, hydrophilic groups of the PBO precursor (OH, NH and CO), which would otherwise adversely affect the dielectric properties and water absorption. This is, for example, an essential advantage of polybenzoxazoles over polyimides and in particular over hydroxy-polyimides. However, the cyclization is not important just for good dielectric properties and low water absorption of the end product, but also for its high heat stability.
The demands made of the cyclized final product are very high. For example, besides the lowest possible dielectric constant and high thermal stability, a particular requirement is for low moisture absorption. This is because absorbed moisture impairs not only the electrical properties, but can also promote corrosion of the metallic conductor tracks or result in bubble formation and flaking at high temperatures. Polyimides and polybenzoxazoles have the following advantages over many other high-temperature-stable polymers:
In contrast to the cyclized end product, they can be applied to a substrate as a soluble precursor and then cyclized, during which the solubility and thus the sensitivity to solvents and other process chemicals decreases greatly. For this reason, the processing of, for example, precyclized polybenzoxazoles is difficult.
The addition of suitable photoactive components to the precursors allows the preparation of photo-sensitive compositions, enabling inexpensive, direct structuring of the dielectric. Polybenzoxazoles have the further advantage over polyimides of being structurable in positive mode and developable in aqueous-alkaline media (see EP 0 023 662 B1, EP 0 264 678 B1 and EP 0 291 779 B1). To this end, the PBO precursors used must be soluble in alkaline developers, preferably ones which are free from metal ions.
Benzocyclobutene (BCB), which can be processed in a similar way and structured negatively, has a significantly lower heat stability than polyimide and polybenzoxazole.
A further, important requirement in connection with inexpensive production of microelectronic components is the planarization capacity of the dielectric. The reason for this is that, during the production of such components, level differences occur during application of various layers, making further processing, for example lithographic production of fine structures, more difficult. By contrast, a planarized substrate allows photo-lithographic processes to be carried out with better dimensional accuracy and greater process tolerances. The use of a dielectric which allows good planarization enables expensive polishing procedures (chemical mechanical polishing, CMP) to be avoided in the production of the components.
Alkali-soluble PBO precursors which are suitable for the preparation of photosensitive compositions are disclosed, for example, in EP 0 023 662 B1, EP 0 264 678 B1, EP 0 291 779 B1 and DE 37 16 629 C2; these precursors can be cyclized on the substrate (in this respect, see EP 0 291 778 B1). However, the known polymers exhibit relatively high moisture absorption, for example 1.7% (see EP 0 291 778 B1), after cyclization (conversion into the polybenzoxazole). There is no mention of the planarization capacity of the polymers prepared.